For understanding the problem underlying the present invention, reference is made to FIGS. 1 and 2 of the drawings. FIG. 1 shows an injection molding system, in schematic representation. From an injection molding machine connected via a feed bushing, hot melt (i.e. molten plastic material) is conducted through the flow channels 4 of a heated hot-channel distribution block 2 to two or more nozzles 10. The injection nozzles 10 are positioned in accommodating bores 8 in the injection-side member 5A of the mold tool 6. The injection nozzles are connected via the so-called mouth (or orifices) 13 to cavities 7, in which the molded parts are formed and which are usually found in the ejection-side part 5B of the tool 6.
There is a considerable difference between the required temperature of the melt and that of the tool. For example, the processing temperature of the melt can be 250° C., while the tool can have a temperature of 50° C. This temperature difference results in a considerable flow of heat away from the nozzles, especially at the points of contact between the nozzles and the tool. This heat loss must constantly be compensated by heating of the nozzles. This heating is effected by means of heating cylinders capable of being attached to the nozzles, to which the present invention relates. The most critical point with regard to the risk of overcooling of the melt is at the mouth 13, since here the heat-absorbing capacity of the surrounding nozzle material is small owing to the gradual reduction in the diameter of the nozzle and the fact that there is an intimate contact between the nozzle and the tool.
These conditions require that heating of the nozzle in the mouth region 12 must be greater than in the shank region 11. The heat supply required for this in the mouth region 12 is also heavily dependent upon the operating conditions, the material of the melt and the tool, and the shape of the mouth 13. In addition, the heat supply must be effectively controllable, since the range of the processing temperature of some melts is very narrow. In addition, in the cold state of the tool for example, a greater quantity of heat must be supplied to the mouth region 12.
It is already known that the nozzles may be equipped with two separate heating cylinders 20A, 20B, as shown, for example, in FIG. 2, specifically, one for the mouth region 12 and one for the shank region 11. The heat conductors of the two heating cylinders are capable of being controlled and/or regulated independently of one another. The heating cylinders typically consist essentially of a hollow supporting cylinder, which is provided with a spiral groove for the accommodation of a heating cartridge, in which a heat conductor is embedded. The pitch of the spiral groove on the supporting cylinder of the heating cylinder for the mouth region 12 is as a rule smaller than the pitch of the spiral groove on the supporting cylinder for the shank region 11. On the outside of the heating cylinder is found a protective metallic jacket 23. In FIG. 2, in addition to the illustration of the nozzle, the accommodating bore 8 in the tool is also indicated.
One disadvantage of this known system is that two separate heating cylinders 20A, 20B are required. An additional disadvantage is that the electrical connection 34 of the heating cylinder 20A for the mouth region lies quite far in front in the direction of the mouth, so that a corresponding longitudinal groove 9 (FIG. 2) must be provided for its accommodation in the bore 8 of the tool-injection-side member 5A. This necessity, apart from the additional expense, may in some tools also result in problems of space. Moreover, a considerable risk of damage to the electrical connection 34 exists upon introduction of the nozzles into the accommodating bores 8 of the tool. In FIG. 2, reference numeral 35A designates the electrical connection to the temperature sensor for the heating cylinder 20B. The corresponding temperature sensor for the heating cylinder 20A is not shown.